Purification of the chloroplast-associated superoxide dismutase of maize

Superoxide dismutase (SOD) isozymes are associated with the cytosolic, mitochondrial and chloroplastic fractions of maize seedlings (Baum and Scandalios, Differentiation 13:133, 1979). The isolation and characterization of the cytosolic and mitochondrial superoxide dismutases has been recently reported (Baum and Scandalios, Arch. Biochem. Biophys. 206:249, 1981). The chloroplast-associated isozyme, SOD-1, is encoded in the nuclear gene Sod1 and is distinct from the cytosolic isozymes (SOD-2, SOD-4) though it is similar to these proteins with respect to its molecular weight, subunit composition, metal content and sensitivity to cyanide (Baum and Scandalios, Differentiation 13:133, 1979; Baum and Scandalios, Arch. Biochem. Biophys. 206:249, 1981; Baum and Scandalios, J. Hered., in press, 1982). SOD-1 has been purified to homogeneity using conventional purification techniques (Table 1). The purity of the isozyme preparation was determined by non-denaturing and sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE). The holoenzyme molecular weight (Mr) of SOD-1 was estimated to be 33,500 ± 1,500 by G-100 Sephadex chromatography. The subunit molecular weight of SOD-1 was estimated to be approximately 14,500 by SDS-PAGE on 15% gels. It was concluded that SOD-1 is a dimer with a molecular weight of 31,000 - 33,000 and that it is composed of apparently equal subunits. Like the cytosolic isozymes, SOD-1 appears to have at least one intrachain disulfide bond per subunit.

Table 1.

J. A. Baum and J. G. Scandalios


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